Organize the source code in modules

[milancurcic]

See #5 (comment).

Let's stick with .f90 for now until VS Code allows overriding defaults.

[rajkumardongre]

Alright, I am currently planning to tackle this particular issue, and once it's done, I will shift my focus to the other two(Address remarks by @interkosmos and Write a few tests for the GET request ) issues. Would that be alright?

[rajkumardongre]

Hello, I have successfully migrated the code of the http-client.f90 file into http.f90, http_request.f90, http_response.f90, and http_client.f90, and also added a simple_get example to demonstrate the working of high level API, to make GET request, in this PR.

[milancurcic]

Done by #10